This invention relates generally to a navigation instrument for use with roadway vehicles such as rally cars, autobuses and trucks operated on a regular run, etc. More particularly, the present invention relates to a navigation instrument having a means for counting up and counting down of travel of the vehicle in order to exactly measure travelling distance even if the driver make miss-course and returns the vehicle in the scheduled course.
Navigation instruments have been incorporated in roadway vehicles for calculating and displaying vehicle running conditions with respect to scheduled ones so that the driver can arrive at the destination on time.
Generally, as shown in FIG. 1, the navigation instruments comprise a first pulse generator 10, a clock pulse generator 12, a first and second counters 14 and 16, an arithmetic circuit 30, a display control circuit 40 and a display unit 50. The first pulse generator 10 provides a pulse every time the vehicle travels a constant distance. The first counter 14 counts the pulses fed from the pulse generator 10. The clock pulse generator 12 generates clock pulses to be fed to the second counter 16 and counts the clock pulses. A schedule speed setting unit 18 and a set/start circuit 20 are incorporated to the arithmetic circuit 30. The schedule speed setting unit sets a scheduled average speed and the set/start circuit 20 releases the reset state of the first and second counters 14 and 16 and starts navigating operation. The arithmetic circuit 30 calculates the average speed of the vehicle, the deviation of the travelling distance from a scheduled distance, and the deviation of the travelling time from a scheduled time in accordance with the contents of the first and second counters 12 and 16 and a scheduled average speed. The display control circuit 40 is responsive to the calculated results to drive the display unit 50 so that the calculated results can be sequently displayed on the display unit.
In such navigation instrument, if the driver drives the vehicle along a wrong course which is out of scheduled course and returns to the scheduled course, the first counter 14 counts the distance of miss-coursed travel as well as distance of travel along the scheduled course. This will cause error of calculation of the average speed of the vehicle, the deviation of travelling distance from a scheduled distance, and the deviation of the travelling time from a scheduled time.